Optical discs and other optical information recording media are becoming increasingly popular as media for recording information. These optical information recording media include recordable CDs (CD-Rs), which are each a light-transmissive resin substrate of 1.2 mm in thickness and 120 mm or 80 mm in diameter on which a reflective layer and recording layer are formed sequentially. In recent years, however, there is a demand for higher information recording density. To meet this demand, methods that use a shorter laser wavelength and object lens having a larger NA (numerical aperture) have been researched and new optical information recording media such as recordable DVDs (DVD±Rs) have been developed. These DVD±Rs adopt a structure where two light-transmissive resin substrates of 0.6 mm in thickness each are attached to each other, with a reflective layer and recording layer sandwiched in between, to increase the tolerance for the disc's tilt to support shorter wavelengths and larger NAs.
In recent years, however, there is a demand for even higher information recording density to record high-definition image data. To meet this demand, optical information recording media that use laser beams of even shorter wavelengths to record and play back information have been proposed. Examples of these optical information recording media include recordable HD_DVDs (HD_DVD-Rs) having a recording capacity of 15 GB per side, and recordable Blu-ray Discs (BD-Rs) having a recording capacity of 25 GB per side.
HD_DVD-Rs adopt a structure where a recording layer and reflective layer are formed sequentially on a resin substrate of 0.6 mm in thickness having a guide groove (pre-groove) formed on one side, after which a protection layer is formed thereon to protect the reflective layer, and then a dummy substrate of 0.6 mm in thickness is attached thereon. This structure itself is the same as that of DVD±Rs. BD-Rs, on the other hand, are structured in such a way that a reflective layer and recording layer are formed sequentially on a resin substrate of 1.1 mm in thickness having a guide groove formed on one side, after which a protection layer constituted by a light-transmissive inorganic material is provided thereon to protect the recording layer, and then a cover layer of 0.1 mm in thickness constituted by a light-transmissive resin is provided thereon, with the resulting layered structure formed to a diameter and thickness equivalent to CD-Rs and DVD±Rs. Recording layers of these optical recording media are constituted by organic materials containing azo dye, cyanine dye or other organic dye or inorganic materials such as Si, Cu, Sb, Te and Ge, and record or play back data by means of a laser beam of 405 nm in wavelength.
If an organic dye is to be used for the recording layer, a dye that provides good recording characteristics under a laser beam of 405 nm in wavelength is selected. Examples of such dye ideal for optical information recording medium include those disclosed in Japanese Patent Laid-open Nos. 2007-45147 and 2007-196661.